1. Field of the Invention
The present invention relates generally to the field of injection pumps and metering devices, particularly for high pressure fluids, such as chemicals and catalysts. More particularly, the invention relates to a technique for metering the flow from a series of reciprocating injection pumps at precise rates based upon closed-loop control of a parameter closely related to actual flow from the injection pump.
2. Description of the Related Art
Many industrial processes require the injection of fluids at very precise rates. For example, in the manufacture of synthetic plastics and other chemical products, high pressure catalysts are injected into process streams to facilitate or accelerate chemical reactions. Because the chemical compositions of the catalysts are often critical to the promotion of the large scale chemical reactions occurring in such processes, their carefully controlled injection into the process stream is often key to obtaining consistent, high quality product. In typical industrial chemical processes catalysts must be injected at precise volumetric or mass flow rates into relatively much larger flows of raw and intermediate products. In addition to promoting the desired chemical reactions, the catalysts often affect important process parameters such as pressure and temperature in process machinery and reaction vessels.
Various apparatus have been proposed and are currently in use for injecting catalysts into chemical process streams. Because many industrial chemical reactions occur at elevated pressures, such structures have been adapted to inject catalyst at very high pressures and precise flow rates. In one known arrangement, a reciprocating plunger-type injection pump is driven by a hydraulic drive cylinder disposed coaxially with the injection pump. The drive cylinder retracts to draw catalyst into the injection pump and extends to express catalyst, via appropriate high pressure valving, into the process stream or reaction vessel. Flow from a hydraulic pump coupled to the drive cylinder is controlled to meter the flow from the injection pump. The hydraulic drive cylinder may be provided with a rod on either side of its piston, permitting it to drive two plunger-type injection pumps on either end of the cylinder. In the latter case, output from the injection pumps is typically combined, via a shuttle valve or similar arrangement, to provide a near steady flow of catalyst as the hydraulic drive cylinder and associated injection pumps continuously reciprocate under the influence of pressurized fluid from the hydraulic pump. Proximity sensors or similar limit switch devices may be associated with the hydraulic drive cylinder or the injection pumps to automatically shift directional control valving between the hydraulic pump and the drive cylinder, causing the drive cylinder and injection pumps to automatically reciprocate between stroke limits.
While control of such systems may be closed-loop with respect to output flow from the hydraulic drive pump, control of the output flow rate of the injection pump itself is typically open-loop. For example, in one known catalyst injection system a drive cylinder is powered by a swash plate-type variable-volume, axial-piston drive pump. The output flow rate of the drive pump may be varied by movement of a swash plate against which a rotating piston set rides. Control circuitry associated with the pump generates a position command for the swash plate based on a desired level of a process parameter, such as temperature. A sensor positioned in the reaction vessel or process stream plumbing provides a feedback signal indicative of the actual level of the process parameter. The swash plate position command is generated by the control circuitry based upon known relationships between the process parameter and catalyst injection rate, the drive pump output flow rate and the swash plate position, and the drive pump output flow rate and the capacity of the drive cylinder (i.e. the effective cross-sectional area of the drive cylinder). The control circuitry regulates the swash plate position in a closed-loop manner, but only so as to maintain the swash plate in the commanded positions. No control loop is closed on the actual output flow from the injection pump, or any parameter directly indicative of the injection rate.
In another known arrangement, a metering valve is provided between the drive pump and the drive cylinder. The metering valve is modulated to control flow into the drive cylinder based upon the desired and actual levels of a process parameter, such as reaction temperature. However, as in the previous case, no control loop is closed on actual injection rate or any parameter closely associated with the actual rate.
Such systems are often incapable of providing sufficiently precise control of high pressure catalyst injection. For example, in the manufacture of polyethylene, reaction vessel pressures in excess of 1,000 bar are not uncommon. Depending upon the throughput of the reaction vessel, precise catalyst injection rates on the order of only several cubic centimeters per minute may be demanded of the catalyst injection pump system. However, very slight variations in the catalyst injection rate may result in dramatic swings in process temperature and pressure. It has been found that catalyst injection systems of the types described above can produce variations in catalyst injection rates from desired levels in excess of tolerable ranges as dictated by equipment operating limits and product quality specifications. Such variations may result from factors such as hysteresis in the reciprocating pump velocities, tolerances in drive pump output flow rate, tolerances in metering valve flow, lack of sufficient repeatability in pump or valve set points and corresponding flow rate, and so forth.
Swings in process conditions resulting from such catalyst injection rate variations can not only lead to the production of poor quality or down-graded product, but can necessitate interruption of the plant process and decompression of reaction vessels in other process stream equipment. In the latter case, significant costs can be incurred from down time to purge the process equipment and bring the process back on line, as well as from repair or replacement of damaged equipment. Moreover, even when variations in catalyst flow rate remain within acceptable limits, improved product could often be obtained if process parameters affected by catalyst injection rates, such as reaction temperature, could be more accurately controlled.
In addition to the foregoing drawbacks, conventional chemical metering in catalyst injection systems do not effectively optimize the use of injection pump drive circuitry. In particular, such systems generally include a hydraulic drive pump for each injection pump. Where a drive pump-based control technique is employed, this is necessary to afford control of the injection pump (such as via control of a swash plate of the drive pump). However, even where metering valves are employed, a separate drive pump is typically provided for each injection pump. Again, this is often the result of the particular control technique employed. Where systems include a bank or series of injection pumps, such redundancy becomes expensive and adds to the complexity of the resulting system.
There is a need, therefore, for an improved pumping and metering system, particularly for injecting chemicals and catalysts of the type used in industrial chemical processing applications. More particularly, there is a need for an improved metering or injection system capable of providing closed-loop control of actual flow rate, or of a parameter closely indicative of flow rate, from a reciprocating injection pump. Ideally, the technique should be capable of being employed in systems closing a nested control loop on a process variable or parameter, such as reaction pressure or temperature. The technique should also be capable of implementation on new chemical injection systems, as well as afford the possibility of being retrofitted to the many injection systems currently in use. The technique would advantageously provide for some reduction in drive circuitry in multiple injection pump systems.